Finely divided solid particles, and/or very small droplets of liquids in a gas (dispersoids) are conventionally separated from the gas in which they are suspended by passing the dispersoid through a batt or septum of woven or nonwoven fabrics of natural or synthetic fibers. Hereinafter, for clarity, the term batt is used to describe a mass of conventional fibers and the term septum is used to refer to a masss of unsintered fibers formed from solid primary particles of PTFE or from agglomerates of primary PTFE particles. Separation of particles larger than about 5 .mu. (referred to as relatively large particles) may be efficiently accomplished by conventional filter elements but these filter elements are insufficiently effective for the removal of less than 5 .mu. and submicron size particles of a dispersoid, for several reasons.
It is well-known that, even with an acceptable initial pressure drop, a conventional batt for the separation of less than 5 .mu. and submicron particles is rapidly plugged and the pressure drop soon becomes intolerable. Heretofore attempts have been made to use electrostatic materials in an air filter, as for example in the aforementioned non-woven fabrics, but these fibers are difficult to distribute evenly, must necessarily be deposited in a batt of considerable thickness and density to be effective with submicron size particles, and therefore, in a filter of this type, air flow is uneven and channeled. Often, as in a batt of non-electrostatic fibers, a batt of electrostatic fibers becomes quickly unusable because the desired air flow cannot be obtained due to severe matting of the fibers, and not because the batt is overloaded with filtered particles. Because, with unsatisfactory gas flow through a batt, separation of very small particles is of no practical significance, dense batts of known very fine fibers have very limited utility and are largely ineffective. Morever, for the separation of many chemicals, known batts are insufficiently inert to either a corrosive gaseous medium or to corrosive particles suspended in a noncorrosive gaseous medium, particularly when separation is to be effected at elevated temperatures.
It is also known that, according to theoretical considerations in filtration, the particle size most difficult to filter is about 0.25 micron or smaller, for perfectly spherical particles. With increasing size, the increasing inertia of the particle acts to cause the particle to impinge on a fiber of the filter medium. With decreasing size, the amplitude of the Brownian oscillation increases and acts to bring the particle more easily into contact with a fiber of the filter. According to this theory, once a submicron particle comes into contact with a fiber it is removed from the gaseous medium being filtered. This is true only if the area of contact between the fiber and particle is sufficiently large to offer enough attractive forces to overcome the inertia of the particle. Conventional non-woven fabrics are incapable of providing a sufficiently large contact area and are therefore insufficiently effective. However, the fibers of fibrillatable PTFE randomly deposited generally horizontally, one overlapping and intertwining another so as to be overlaid on a support, are surprisingly effective to provide a sufficiently large contact area and are therefore efficient filters especially for submicron size particles.
In particular, it is known that glass fibers and fibers of various polyolefinic resins are effective in the form of a batt or woolly mass to remove liquid or solid particles where the particles are relatively large in size, in the range from about 5 .mu. to about 100 .mu.. For example, a batt of very fine glass fibers may be confined between parallel spaced apart screens, each glass fiber having a diameter in excess of about 10 .mu.. For another example, where a fabric of polytetrafluoroethylene fibers supported on a wire meshs screen is used, the fibers are spun with predetermined uniform diameters in excess of about 50 .mu. and then sintered, and the sintered fibers are relatively coarse, being several times the diameter of unsintered fibers of PTFE.
Thus there are presently few available filtering media which are effective in removing submicron size particles; still fewer are able to remove contaminant submicron particles while maintaining sufficient permeability to gas flowing at a relatively rapid rate. Most synthetic fibers become wet with aerosols or liquid-containing smokes, are easily channelled and matted, or liquid particles tend to coalesce and tend to plug the filter. Only polyfluoroolefin fibers are able to withstand prolonged exposure to corrosive liquid-type smokes or aerosols at a temperature in the range from about 200.degree.C to about 300.degree.C. without soon becoming ineffective.